1. Field of the Invention
The present invention relates to a surface acoustic wave device and more particularly, to a surface acoustic wave device having an excellent temperature characteristic and a large electromechanical coupling coefficient.
2. Description of the Related Art
Surface acoustic wave devices have been widely used for passband filters in mobile communication equipment. One of the surface acoustic wave devices known in the art is a surface acoustic wave resonator or a resonator filter having a structure in which an interdigital transducer (referred to as an IDT hereinafter) comprising comb-shaped electrodes and reflectors is provided on a piezoelectric substrate. Piezoelectric single crystals such as lithium niobate, lithium tantalate, quartz and PZT-based piezoelectric ceramics are used for the materials of the piezoelectric substrate of a surface acoustic wave resonator or a resonator filter.
In order to obtain good characteristics in the surface acoustic wave device, materials having good temperature characteristics of group delay time (referred to TCD hereinafter), or materials having small characteristic variations due to temperature changes, are thought to be necessary. Lithium niobate described above has a poor TCD while a TCD of lithium tantalate was not so good compared to that of Lithium niobate. Since quartz is well known to have a good TCD, a variety of the surface acoustic wave devices have used quartz to form a substrate of the surface acoustic wave device.
Although the quartz substrate has a good TCD, its electromechanical coupling coefficient (abbreviated as K.sup.2 hereinafter) is relatively small when, for example, a Rayleigh wave is excited in a surface acoustic wave device in which an IDT was disposed on the quartz substrate.
Accordingly, there are various problems associated with a low K.sup.2 of a quartz substrate. Specifically, when a surface acoustic wave resonator or a surface acoustic wave resonator filter are constructed by disposing an IDT and reflectors on the quartz substrate, a lot of electrode fingers for forming the reflectors are required as compared with the case when the substrate is constructed of materials other than quartz, thereby causing the surface acoustic wave resonator or a surface acoustic wave resonator filter to be significantly larger in size. In addition, the operation band of the surface acoustic wave resonator is narrowed and it is difficult to obtain a wide band characteristic. It is also a problem that the excitation efficiency of the surface acoustic wave is low, which results in a large insertion loss because the excitation resistance of the surface acoustic wave resonance element becomes large when the excitation efficiency for the surface acoustic wave is low.
Moreover, when a transversal type filter is constructed by providing two or more IDTs on the quartz substrate, the insertion loss of the surface acoustic wave filter and surface acoustic wave resonator filter becomes larger than the case when the substrate is constructed of other materials. The low level of K.sup.2 of the quartz substrate also results in a narrow passband of the surface acoustic wave filter as compared with the case when the substrate is constructed of other materials.
A leaky surface acoustic wave device comprising a quartz substrate also suffers from the aforementioned problems.